1. Field of the Invention
The present invention is generally related to phototransistors and, more particularly, to a shunt phototransistor circuit which provides an increased level of protection in the event of a reverse voltage condition.
2. Description of the Prior Art
Phototransistors are well known to those skilled in the art. Since phototransistors are generally more sensitive than photodiodes to changes of illumination, they are preferred as switching devices in some applications where fairly small changes in the level of illumination are expected. The phototransistor works both as a PN junction photodetector and an amplifier of the current generated by incident light. Phototransistors, like photodiodes, respond to light energy that affects the silicon lattice near and within a PN junction. The frequency of the light must be in the correct range. In other words, the energy of the photons must be sufficient to break the covalent bonds within the semiconductive material. For silicon, light in the near infrared and visible regions of the spectrum will cause this type of response.
Some phototransistors are provided with a shunt resistor that establishes a threshold optical signal level below which the light is insufficient to provide an electrical signal. As will be described in greater detail below, the use of a threshold in this manner is beneficial, particularly in reflective sensors where the threshold provides an enhanced contrast ratio between light and dark surfaces. In certain other applications, the threshold level can be useful for preventing turn on by leakage current at elevated temperatures. When phototransistors are fabricated with integrated shunt resistors, their use is limited in certain applications since excessive current may flow when these devices are connected with reverse bias voltage across the collector and emitter of the transistor.
It would therefore be beneficial if a phototransistor could be fabricated with a shunt resistor and a means for preventing damage that could otherwise be caused by a reverse voltage placed across the collector and emitter of the transistor.